Abstract : This project was originally established for the purpose of modifying the standard diver's non-return valve to overcome three objections: (1) the upper valve stem guide, in some cases, became deranged to the extent of binding the valve stem; (2) manufacturers were unable to provide a valve which would meet current specifications, i.e.: open with less than one psi and seat with any back pressure from 2-inches of water to 350 psi; and (3) the standard stem washers could not be readily replaced in the field. Subsequent to the commencement of work on this project, a new valve was received from bureau of Ships, Code 588. This valve was evaluated and certain modifications were made on it. This report describes the modifications made to both valves and covers the objective and subjective tests conducted. Objective test results are tabulated and/or graphically illustrated. (Author)